1. Field of the Invention
The present invention refers to a composition of matter based on particular unsaturated polyester resins, particularly suitable for solidifying liquid or solid low or medium level radioactive wastes.
The present invention further refers to articles obtained by cross-linking aqueous emulsions comprising said composition of matter and radioactive material.
Said solid articles are characterized by their stability to radiation and chemical agents for highly prolonged periods of time, by low susceptibility to leaching if immersed in water or in aqueous solutions, and by high mechanical resistance, in particular impact and compression resistance, whereby said articles may be transported and stored without danger that the radioactive substances therein contained may be released therefrom, even in case of accidents or natural calamities.
The present invention further refers to a process suitable for obtaining the articles based on the aforesaid composition of matter.
2. Prior Art
Processes for solidifying said low and medium level radioactive wastes in the form of solid articles are known, which articles are obtained by immobilizing said radioactive wastes in concrete, cement or in particular in resins or bitumen.
The articles obtained by said methods, however, have considerable drawbacks. Thus, for instance, the solidification by concrete or cement which is obtained from the suspension of cement and radioactive solutions and successive hardening, cannot be carried out in the case that the radioactive wastes contain particular chemical substances which act as retardants or inhibitors in the hardening phase, as for instance borates. Further in the presence of other chemicals such as for example the sulphates, the quality of the articles obtained is not satisfactory. Another drawback is due to the fact that the pieces in concrete or cement thus obtained are not sufficiently resistant to leaching of the radionuclides contained therein. This occurs because, as well known, concrete and cement normally have highly porous structure through which water, which has accidentally come into contact with such a piece, may penetrate into the piece, whereby a part of the radioactivity is leached therefrom. Finally, the solidification by concrete or cement does not permit significantly to reduce the volume of the residues to be treated, in as much as, in majority of the cases, it is not possible to solidify by concrete or cement more than 20%-30% of the radioactive waste with respect to the volume of the final product.
The use of urea-formaldehyde resin emulsions and radioactive wastes, chemically hardened, has also been proposed. Said process, while it does not permit to overcome the aforementioned problem of the leaching of radioactivity, has, further, the drawback that a piece thus obtained has low mechanical resistance, so that there is a danger that product may become cracked in the case of mechanical accidents, for example during the transport and storage operations. As to the process of solidification by bitumen, in spite of the fact that it offers better results with reference to the resistance to leaching, cost problems are associated thereto, since the solidification must be carried out at a temperature above 150.degree. C. in expensive mixing apparatus. Furthermore, safety problems, especially in the case of the treatment of solutions containing nitrates, may occur which may give rise to bitumen oxidation, which sometimes involves explosive reactions. Finally, the mechanical characteristics of the product obtained by solidification in bitumen are not wholly satisfactory, and considerably deteriorate as the storage temperature increases. In view of the solution of said problems, the art has directed its attention to the use of unsaturated polyester or epoxy resins for the solidification of low and medium level radioactive wastes in the form of solid articles, as described in French Patent Application No. 7425298, and in the form of emulsions in polyester resins of the "water in oil" type, as described by Subramanian and others and as reported in U.S. Pat. No. 4,077,901. In every case, however, it has been noted that insufficient attention has been devoted to the correct choice of an unsaturated polyester resin which should yield optimum result as concerns the most significant characteristics which solid articles incorporating radioactive substances must present.
The literature nearly always discloses for this purpose conventional polyester resins based on phthalic anhydride, maleic anhydride and glycols, and in some cases vinyl ester type resins are proposed. No particular effort has been devoted to discover resins suitable to the purpose of obtaining the optimum characteristics required to the solid articles nor resins more suitable to the preparation of emulsions of an unsaturated polyester resin and a radioactive solution or suspension. It is known that unsaturated polyester resins may in general be emulsified with water in the presence of suitable surface active agents or electrolytes dissolved in the water itself or even without their help. In suitable conditions emulsions may be formed both of the "water in oil" type and of the "oil in water" type. The preparation of resin/water emulsions is well known to persons skilled in the art, as described for instance in U.S. Pat. Nos. 3,442,882 and 3,792,006.
The unsaturated polyester resins heretofore proposed for the purpose of solidifying solid or aqueous low and medium level radioactive wastes, have been proved not to be suitable, both as concerns the characteristics required from the solid articles obtained therefrom and the emulsifying of the radioactive wastes in the resins themselves.